Naturally I got interested in the technical aspects of the cave rescue in Thailand, particularly communications. They used the "Heyphone", a voice radio designed by a UK radio ham, John Hey, G3TDZ, as open-source hardware specifically for cave rescues. It uses upper (single) sideband voice on 87 kilohertz in the VLF (very low frequency) band. (That's what it says -- 87 kHz is actually LF).

The "antenna" consists of two stakes driven into the ground about 20m apart. Enough of the current between them fringes outward to couple to another antenna up to a few hundred meters away (or down).

John Hey passed away in 2016 so he didn't get to see his work used here. But ham radio should get some of the credit.

What is the HeyPhone?. The HeyPhone was designed by John Hey with the support of the British Cave Rescue Council BCRC and the Cave Radio & Electronics Group (CREG), which is a special interest group of the British Cave Research Association (BCRA). In the period up to 2001, working with CREG, the BCRC issued over fifty HeyPhones to the UK's cave rescue teams, allowing communication between the surface controller and one or more underground teams. This was the first time that a national organisation had systematically equipped its rescue teams with such advanced equipment. The impact of the HeyPhone on the UK cave rescue service was not due to any novel features of the HeyPhone - in fact, none are claimed - but to the dedication of John Hey and the CREG team that sought to bring the project to fruition by design, experimentation and documentation.